When a large part of bone is lost, employed as a medical treatment is bone autotransplantation in which a piece of bone is removed from a patient's own normal bone and grafted into the bone defect, or artificial bone implantation in which a piece of artificial bone made of an artificial material is implanted into the defect. However, a limited amount of harvested bone imposes limitations on bone autotransplantation. Besides, bone autotransplantation falls heavily on the body of the patient because normal cells are damaged during surgery. In addition, because the removal of a piece of bone to be utilized for autologous bone grafting from a patient's own normal bone creates a new defect in the bone, this method cannot be an essential treatment when the amount of missing bone is large. On the other hand, artificial bone implantation does not have problems that bone autotransplantation has, because the former employs industrially manufactured artificial bone. However the mechanical and biological properties of artificial bone are different from those of natural bone, the properties of an artificial bone place limitations on the use thereof. For example, artificial bone made of metal materials such as titanium alloy, which normally have high strength while having a large coefficient of elasticity and lacking toughness, causes stress shielding due to differences in mechanical properties between the metal material and the surrounding bone, when the artificial bone is implanted in parts that are continuously under great load. Another problem of artificial bone is that it is not directly integrated into natural bone. On the other hand, artificial bone made of bioceramics such as hydroxyapatite, is usually highly biocompatible as well as highly bioactive and excellent in binding with natural bone, while it is weak against external impact. Thus it is not suitable to use at parts or places that tend to receive large load in a moment.
The selection of polymers such as ultra high molecular weight polyethylene for a material of artificial bone solves the problems that metal materials and bioceramics have. In particular, polyetheretherketone, which is often abbreviated to PEEK, has mechanical properties close to the mechanical properties of natural bone, and PEEK is also excellent in biocompatibility. Therefore its adaptation to orthopedic materials used at parts that require high strength is expected. Furthermore, artificial bone made of a combined material of polymer and bioceramics with biological activity, capable of directly binding with the native bone, has been developed.
On the other hand, it is well known that the structure of artificial bone, as well as chemical properties such as biocompatibility and biological activity and physical properties such as strength and elastic modulus, is an important factor from the viewpoint of binding capability with the native bone. Developed are many artificial bones whose surface or entire structure is made porous to facilitate penetration of the biological tissue into the inside thereof when it is implanted in the body of a living organism. Innovative approaches to providing artificial bone made of polymeric materials including PEEK with a porous superficial layer or a convexo-concave surface have been made in order to make the best use of its strength and to make it have binding capability with native bone.
Patent document 1 discloses an artificial acetabular cup including a lining layer, which has a porous structure made by sputtering PEEK particles with a plasma torch, against a bearing superficial layer made of a composite material of PEEK and carbon short fibers.
Patent document 2 teaches a sponge-like structure formed by a plurality of polymer sheets each with at least one aperture wherein the polymer sheets are stacked up and stuck together with the locations of the apertures shifted with each other.
Patent document 3 discloses an orthopedic tool, capable of being implanted, having a porous organic polymer layer with desired pores. The method of forming the porous organic polymer layer comprises embedding a pore-forming agent in a polymeric material, allowing the organic polymer to contact a solvent for eluting the pore-forming agent in order to make the solvent to elute the agent and form desired pores.
Patent document 4 teaches an orthopedic implant formed from a thermoplastic resin with a convexo-concave surface wherein the convexities and concavities are formed by etching, sand blasting, grinding or other methods.
Patent document 5 discloses a method of heat molding a thermoplastic material with a mold having concavities and convexities in the inner walls thereof.
Patent document 6 teaches a method of decalcomania transferring of concavities and convexities in the surface of a surgical implant comprising press-fitting an acid-soluble metal plate with a predetermined shape into the surface of the surgical implant made of a thermoplastic resin, and dissolving the acid-soluble plate.
However, the conventional methods and products have defects: Some require expensive apparatuses, like the product of patent document 1; others call for sheet materials to form a porous structure in addition to a polymeric material for parenchyma to realize the strength of artificial bone, like the product of patent document 2; or still others need preparation of polymers including a pore-forming agent, like the product of patent document 3. It is easily supposed that the method of patent document 4 does not provide sufficient concavities and convexities suitable to let biological tissue penetrate into the implant. Furthermore, the methods taught in patent documents 5 and 6 are not suitable to form a porous structure in the surface of polymeric material with a complicated shape.
Also known are methods of making a polymeric material foam thereby forming a porous structure. Among them, one well-known method includes steps of dispersing a solvent with a low boiling point as a foaming agent in a high-molecular weight compound, and heating the high-molecular weight compound with the low-boiling-point solvent dispersed therein to volatilize or decompose the solvent and then to generate gas, thereby forming a lot of foam inside the high-molecular weight compound.
Patent document 7 teaches another method including steps of dissolving an inert gas, such as nitrogen gas or carbon dioxide gas, in a thermoplastic resin under a high pressure; releasing the pressure; and heating the thermoplastic resin to a temperature close to its glass transition temperature, thereby making the gas dissolved in the thermoplastic resin foam to produce a porous material.
As understood, all of the aforementioned methods are those to make a high-molecular weight compound in its entirety a porous structure. Such a porous structure includes pores dispersed all over the structure, which may lower the strength thereof depending on the diameters of the pores and the porosity. Therefore the methods are not suitable to produce implants to be used at parts that require high strength.
Patent document 1: JP 2006-158953 A
Patent document 2: JP 2006-528515 T
Patent document 3: JP 2004-313794 A
Patent document 4: JP 2001-504008 T
Patent document 5: JP H2(1990)-5425 B
Patent document 6: JP H4(1992)-20353 B
Patent document 7: JP H6(1994)-322168 A